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Planetary science

Planetary science

Earth-like planets form in the wake of giants

08 Sep 2006 Hamish Johnston

Simulations have revealed that Earth-like planets can be formed with some help from a wandering giant. Astrophysicists in the US have shown that planet formation can occur after a giant planet has migrated through the “habitable zone” – the orbital region where Earth-like planets could exist (Science 313 1413).

Giant planet

Giant migration is believed to be a very common phenomenon and some astronomers think that it suppresses the formation of Earth-like planets by either capturing protoplanetary material or causing the material to be ejected from the habitable zone. Instead, the simulations suggest that migration could further the creation of Earth-like planets by concentrating water and heavier materials into the habitable zone.

The process involves a large Jupiter-like planet forming some distance from a star and then migrating inwards. This takes it through a belt of protoplanetary material, which is coalescing into larger solid objects and ultimately planets like Earth. Astronomers are currently aware of at least 200 planetary systems containing giant planets – 40% of which may have undergone a migration. As a result, more than one third of all known planetary systems could include Earth-like planets.

Sean Raymond of the University of Colorado, Boulder and Avi Mandell of Pennsylvania State University simulated the migration of a Jupiter-sized planet through a disk containing 17 earth masses worth of protoplanetary material. The disk extended from 0.25 to 10 astronomical units (AU) from a star. Earth’s orbit is 1 AU from the Sun.

The migration process lasted 100 000 years, beginning with the giant at 5 AU and ending at 0.25 AU. The simulation covered a further 200 million years with the giant orbiting at 0.25 AU as a “Hot Jupiter”. Four simulations were performed and two resulted in the formation of a planet about the same size as Earth and orbiting within the habitable zone (0.8-1.5 AU) where life is possible. These planets have a much higher water content than Earth, and would have oceans several kilometres thick.

The simulations also resulted in the formation of other planets outside of the habitable zone, including so-called “Hot Earths”, which are much closer to the star than Earth is to the Sun. A Hot Earth was discovered by astronomers in 2005

Both NASA and the European Space Agency will be launching new space telescopes designed to search for Earth-like planets. The simulation results could help astronomers find suitable targets within known giant planet systems.

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